In order to provide reliable communication services, SONET (“Synchronous Optical Network”) networks usually include mechanisms for protecting against optical fiber failures. SONET networks using ADMs (“Add Drop Multiplexers”) are often constructed in the form of rings. Some of these optical rings, such as BLSR (“Bi-Directional Line Switching Ring”), can protect against both a line failure and a span failure.
An example of a SONET BLSR with four ADMs is illustrated in FIG. 1. The BLSR 100 includes ADMs, 110, 120, 130 and 140. Four optical lines connect any two ADMs. Eight spans SP1 to SP8 connect the ADMs to form a bi-directional ring. Each of spans SP1 to SP8 includes one working line and a corresponding protection line. Each of the ADMs adds and/or drops signals to switch data from one span (SP1 to SP8) to another. For example, spans SP1 and SP5 interconnect ADMs 110 and 120 and include working lines carrying data in opposite directions. The protection lines may be used to transmit data in the event an associated working line fails. Data transmitted in such a ring typically must conform to a particular protocol.
BLSR 100 can be configured to provide protection for transmission of data in two ways. First, if a working line fails, the corresponding protection lines may be used. In the alternative, if working lines fail between two ADMs, any communication route directed through the failed line may be rerouted by the ADMs using a process known as span switching. For example, if the working lines between ADM 110 and ADM 120 fail, instead of using the corresponding protection lines, communications may be sent from ADM 110 to ADM 120 via ADM 140 and 130.
Typically, the working and protection lines are provided in a fiber optic bundle. Accordingly, if the working line fails, due to a fiber cut, for example, the corresponding protection line often will also fail. Span switching is thus often preferred to simply switching data from a faulty working line to the protection line.
When ADMs are used for optical line switches, BLSR is one of the preferred sub-network structures. However, a network constructed with switches that are capable of running both routing and signaling protocols, BLSR usually is not the preferred sub-network structure; a mesh structure is generally preferred. For these kinds of networks, there is a need to develop more robust network protection schemes for protecting as against optical line and span failures in the network.